1. Field of the Invention
The invention relates to a frequency switching method and a voltage controlled oscillator using this method, and more particularly to a frequency switching method suitable for a voltage controlled oscillator for driving a spreading code generator by a reverse spread spectrum means in a receiver for spread spectrum communication and a voltage controlled oscillator using this method.
2. Description of the Related Art
FIG. 10 is a block diagram showing a receiver for spread spectrum communication by using a conventional voltage controlled oscillator. In the drawing, 101 denotes an antenna, 102 an amplifier, 103 a mixer, 104 an amplifier, 105 a reference signal source, 106 a crystal resonator for the reference signal source 105, 107 a reverse spread section, 108 a spreading code generator, 109 a voltage controlled oscillator for generating a clock to drive the spreading code generator 108, 110 a crystal resonator for the voltage controlled oscillator 109, 111 a phase error detector for detecting a phase error between a spreading code for spreading a reception signal and a spreading code generated by the spreading code generator 108, 112 a baseband processing section for the received signal, 113 a crystal resonator for generating a clock used in the baseband processing section, and 114 an output terminal. A reverse-spread section 120 for making reverse spread spectrum of the received signal comprises the reverse spread section 107, the spreading code generator 108, the voltage controlled oscillator 109, and the phase error detector 111.
The signal received by the antenna 101 is converted its frequency by the mixer 103 so as to have a frequency band lower than that of the received signal, subjected to the reverse spread spectrum by the reverse-spread section 120, undergone a predetermined processing by the baseband processing section 112 and output to the output terminal 114.
The receiver for spread spectrum communication by using the conventional voltage controlled oscillator 109 shown in FIG. 10 uses the three crystal resonators 106, 110, 113. Even if a circuit is integrated, these crystal oscillators remain as external parts for the integrated circuit. Therefore, in order to make the receiver small and inexpensive, it is necessary to use a plurality of crystal oscillators in common and decrease the necessary number of crystal oscillators.
However, in the receiver for spread spectrum communication using the voltage controlled oscillator 109, the voltage controlled oscillator 109 for generating a drive clock of the spreading code generator 108 must have a variable oscillation frequency and cannot be commonly used for the crystal resonator 106 for the reference signal source 105 and the crystal resonator 113 for generating a clock used by the baseband processing section which need a reference signal having a fixed frequency. Therefore, it is disadvantageous that the crystal resonator 106 and the crystal resonator 113 need to be provided separately.
It is an object of the invention to use a crystal oscillator which configures a voltage controlled oscillator having a variable oscillation frequency also as a crystal oscillator of an oscillator for generating a fixed frequency to decrease a necessary number of crystal oscillators, thereby making a receiver compact and inexpensive.
To achieve the above object of the invention, control input is integrated, when it is detected that the integrated value exceeds a predetermined range, a frequency division ratio used to divide the frequency of a reference signal is changed by one cycle, and the integrated value is reset to a predetermined initial value. Thus, based on a fixed reference signal, a frequency divided output having a frequency variable by the control input is obtained.
Thus, when the divided output is used as the voltage controlled oscillator of the receiver for spread spectrum communication, a crystal oscillator for the voltage controlled oscillator which generates a drive clock for a spreading code generator can be also used as a crystal oscillator for a reference signal source to be a reference signal having a fixed frequency and a crystal oscillator for generating a clock used in a baseband processing section. Accordingly, a required number of crystal oscillators can be decreased, and the receiver can be made compact and inexpensive.